In the state of the art, various methods are already known for assembling two parts made of thermoplastic material, in particular by heat-sealing tongues, by riveting, and by spot welding.
When one of the assembled parts is a trim part, those methods can lead to problems because of shrink marks appearing on the outside face of the trim part, particularly when said trim part is of small thickness.
Indeed, firstly, spot welding leads to a puddle of molten material in the thickness of the trim part, and when this puddle of molten material solidifies, a shrink mark appears in the outside face.
Secondly, riveting and heat-sealing tongues require the trim part to have ribs, and they too lead to shrink marks in the outside surface.